1. Field of the Invention
Embodiments of the present invention relate generally to optical communication systems and, more particularly, to an optical device for wavelength division multiplexing applications.
2. Description of the Related Art
In a wavelength division multiplexing (WDM) optical communication system, information is carried by multiple channels, each channel having a unique wavelength. WDM allows transmission of data from different sources over the same fiber optic link simultaneously, since each data source is assigned a dedicated channel. The result is an optical communication link with an aggregate bandwidth that increases with the number of wavelengths, or channels, incorporated into the WDM signal. In this way, WDM technology maximizes the use of an available fiber optic infrastructure; what would normally require multiple optic links or fibers instead requires only one.
In WDM optical communication systems, it is often necessary to add, drop, or attenuate optical channels within a light beam. This can be achieved by a wavelength selective optical switching device, which directs an input light beam to one of multiple output optical paths. For example, in a 1×2 wavelength selective optical switching device, the optical channels contained in an input light beam enter through an input fiber and are directed to one of two output fibers. There are also more complicated optical switching devices, such as 2×2, 1×N, and N×N switching devices, which are realized by combining several 1×2 devices. In some optical networks, the individual wavelength channels of a WDM input signal are directed to different output fibers by a wavelength selective optical switching device, also known as a wavelength router. Different types of optical switching devices known in the art include wavelength selective switches (WSSs) optical add-drop multiplexers (OADMs), wavelength blocker and dynamic gain equalizers (DGEs).
WDM switching devices commonly include multiple free-space optical systems. These free-space optical systems consist of lenses, mirrors, and other elements, which are optically coupled by regions of vacuum or atmospheric pressure. The optical systems of a WDM switching device must be precisely aligned, since misalignment of these optical systems can cause significant signal loss in the WDM switching device.
FIGS. 1A, 1B are ray tracing diagrams schematically illustrating the sensitivity to positional displacement of an optical system 100 commonly used in the art. Optical system 100 includes a collimating lens 102, a light entry point 101 and a light exit point 105 positioned on a first side of collimating lens 102, and a reflective element 103 positioned on a second side of collimating lens 102. For simplicity, light entry point 101 is illustrated as essentially a point source of light, such as an input port that is optically coupled to an optical fiber, and directs input rays toward collimating lens 102. Collimating lens 102 optically couples light entry point 101 to reflective element 103.
In FIG. 1A, light entry point 101 of optical system 100 is located at front focal point 109 of collimating lens 102 and therefore is also positioned on optical axis 104, which is an ideal scenario. Input rays 110A-C originate from light entry point 101, which is coincident with front focal point 109. Therefore, collimating lens 102 directs input rays 110A-C to reflective element 103 as collimated rays, i.e., light rays that are parallel to optical axis 104. Because of this, input rays 110A-C have normal incidence on reflective element 103, and output rays 111A-C follow return paths that are identical to the incoming paths of input rays 110A-C, as shown.
In practice, perfect alignment of light entry point 101 with optical axis 104 is problematic. Even if optical system 100 is precisely assembled, misalignment between light entry point 101 and optical axis 104 may still occur due to the relative motion caused by thermal expansion or other mechanical deformation of optical system 101 during operation. In FIG. 1B, light entry point 101 is displaced from optical axis 104 a distance 108 along the y-axis as a result of such deformation. Because light entry point 101 is not positioned coincident with front focal point 109, collimating lens 102 does not direct input rays 120A-C toward reflective element 103 as collimated rays parallel to optical axis 104. Instead, input rays 120A-C have a non-normal angle of incidence θ with reflective element 103, as do output rays 121A-C. Output rays 121A-C therefore do not follow paths coincident with input rays 120A-C, so that light exit point 105 is displaced a distance 107 from front focal point 109 along the y-axis.
The magnitude of distance 107 is a function of distance 108 and the distance 106 between collimating lens 102 and reflective element 103. Because of this, when distance 106 is significantly greater than front focal length 113, even a small value for distance 108 may result in a relatively large value for distance 107. Large displacements between light entry point 101, light exit point 105, and optical axis 104 are undesirable since substantial signal loss can result. For example, when light exit point 105 is intended to correspond to an optical output port positioned on optical axis 104, output rays 121A-C may partially or completely miss the output port, depending on the magnitude of distance 107. FIG. 1B only illustrates displacement of light entry point 101 and light exit point 105 from optical axis 104 in the y-z plane, as indicated. In addition, misalignment of light entry point 101 from optical axis 104 may occur along the x-axis (not shown), which can result in further displacement of light exit point 105 and, therefore, additional signal loss. Hence, optical system 100 is inherently sensitive to the small but unavoidable displacements from optimal alignment of light entry point 101 with the other components of optical system 100.
Accordingly, there is a need in the art for an optical system used in communications networks that is less sensitive to positional alignment over prior art systems.